In one aspect, the invention relates to peak temperature logging while drilling a well bore in the earth. In another aspect, the invention relates to a temperature recorder which can be used to log temperatures while drilling a well bore in the earth.
It is sometimes very important to determine the peak temperature of drilling fluid as it is circulated during the drilling of a well. In the drilling of oil and gas and geothermal wells, the drilling fluid is usually a mud which contains bentonite. The drilling fluid is circulated from the earth surface down the drill pipe and through openings in the drill bit adjacent the bottom of the well. The drilling fluid returns in the annulus between the drill pipe and the sidewall of the well to the earth surface. During the drilling operation, the peak temperature in the circulating well fluid does not occur at the bottom of the well adjacent the drill bit but rather in the annulus surrounding the drill bit at some distance from the bottom of the well. When drilling is halted, the peak temperature usually occurs at the bottom of the well. The highest temperature to which the drilling fluid is exposed is determined by equilibrium, taking into account the temperature and circulation rate of the drilling fluid.
Knowledge of the temperature in the formation can be very important. A high geothermal temperature history of a formation over the course of time can produce relatively undesirable viscous crude and tars. A low geothermal temperature history can fail to provide liquid petroleum deposits. Where a geothermal energy source is the drilling objective, the rock temperature is directly related to the energy obtainable from the source. Further, serious damage to the drilling fluid can occur as a result of continued circulation at a temperature where thermal degradation results. When it is known that the thermal limits of the drilling fluid are being approached, steps can be taken to protect the circulating drilling fluid against thermal deterioration.